1. Field of the Invention
The present invention relates to a thin-film bulk acoustic resonator (FBAR) and fabrication method therefore. More particularly, the present invention relates to an air-gap type thin-film acoustic resonator including a substrate having an air-gap formed in an upper portion of the substrate and a fabrication method therefor, wherein the air gap is formed by etching the substrate through a via hole extending into the substrate from a lower surface thereof.
2. Description of the Related Art
Recently, mobile communication devices such as mobile phones have become widely used, and there is a need for smaller-sized and light-weight filters for use in such devices. FBARs are known as being suitable for such small-sized and light-weight filters, and are advantageous in that they can be mass produced at minimum cost and in small size. Further, such FBARs are advantageous in that they can be fabricated to have a high quality factor Q which is a primary characteristic of such filters, for use in micro-frequency bands, and, particularly fabricated to cover even bands for personal communication systems (PCSs) and digital cordless systems (DCSs).
In general, the FBAR device is fabricated by forming a lower electrode, a piezoelectric layer, and an upper electrode which are deposited in order over a substrate. The operation principle of the FBAR device is as follows: Resonance is generated by applying electric energy to the electrodes to induce within the piezoelectric layer an electromagnetic field which varies with time to generate bulk acoustic waves in the vibration direction of the resonance part.
FIG. 1A is a cross-sectioned view of a Bragg-reflector FBAR which is a type of FBAR. In FIG. 1A, the Bragg-reflector FBAR has a substrate 10, reflection layers 11, a lower electrode 12, a piezoelectric layer 13, and an upper electrode 14. In the Bragg-reflector FBAR, acoustic waves generated from the piezoelectric layer 13 are not propagated in the substrate direction, but are reflected from the reflection layers 11, so that effective resonance can be generated. In its fabrication process, first, substances having a large acoustic impedance difference therebetween are deposited over the substrate 10 to form the reflection layers 11, and then the lower electrode 12, piezoelectric layer 13, and upper electrode 14 are deposited in order, so that a resonance part is formed over the reflection layers 11. The Bragg-reflector FBAR is sturdy in structure and does not exhibit stress upon being bent, but has a drawback in that it is difficult to precisely form more than four reflection layers for total reflection and much time and expense is required for its fabrication.
Thus, the air-gap type FBAR has been investigated which uses an air gap instead of a reflection layer, to isolate the substrate from the resonance part. FIG. 1B and FIG. 1C are cross-sectional views which illustrate the structure of a conventional air-gap type FBAR.
The FBAR having a structure shown in FIG. 1B has an air gap 21 under the resonance part formed with a lower electrode 23, a piezoelectric layer 24, and an upper electrode 25 deposited in order, isolating the resonance part from the substrate 20. In the process of fabricating the FBAR, first, a sacrificial layer (not shown) is deposited and patterned over the substrate 20, so predetermined portions of the layer remain on the substrate 20. Next, an insulation layer 22 is deposited on a sacrificial layer and substrate 20, and a lower electrode 23, piezoelectric layer 24, and upper electrode 25 are deposited in order to form a resonance part. The insulation layer 22 serves as a membrane layer supporting the resonance part. Finally, the sacrificial layer is removed to form an air gap 21. That is, a via hole is formed from the outer surface of the substrate to the inner sacrificial layer, and the sacrificial layer is removed by injecting etching solution through the via hole, so that an air gap 21 is formed in place of the sacrificial layer. Further, U.S. Pat. No. 6,355,498 discloses applying an anti-etching material on the substrate 20 which enables the air gap to be adjusted in size and position when fabricating an air-gap type FBAR having a structure as shown in FIG. 1B.
However, such FBAR fabrication process is complicated because a sacrificial layer is needed. Further, the filter design is restrained since the via hole has to be formed on the membrane layer around the resonator. Further, chemical damage can occur to the resonance part since etching is performed through a via hole formed just near the resonance part.
On the other hand, FIG. 1C is a cross-sectional view of the air-gap type FBAR disclosed in U.S. Pat. No. 6,060,818. In FIG. 1C, a photoresist layer is used to form a cavity 35 when etching is applied to a predetermined portion of the substrate 30. Next, an insulation layer 31 is deposited over the entire upper surface of the substrate 30 on which the cavity 35 is formed. Next, after filling a sacrificial substance in the cavity 35, the lower electrode 32, piezoelectric layer 33, and upper electrode 34 are deposited in order over the sacrificial layer and insulation layer 31 to form a resonance part. Next, a via hole is formed through the insulation layer 31 near the resonance part, and the sacrificial substance is etched away through the via hole to form the air gap 35. However, use of a sacrificial substance makes the process complicated, and the resonance part is subject to chemical damage as well due to the etching. Further, part of the insulation layer can remain below the resonance part, which can degrade resonance characteristics.